perf: Prevent enabling of already enabled perf event

Currently, on every --enable perf_event command,
a new event->fd is created and counting of perf
event counter starts from zero and previous
event->fd is lost. This patch prevents this
behaviour.
Signed-off-by: NNitesh Konkar <nitkon12@linux.vnet.ibm.com>

src/util/virperf.c

@@ -198,6 +198,9 @@ virPerfEventEnable(virPerfPtr perf,
     if (!event || !event_attr)
         return -1;
 
+    if (event->enabled)
+        return 0;
+
     if (event_attr->attrType == 0 && (type == VIR_PERF_EVENT_CMT ||
                                        type == VIR_PERF_EVENT_MBMT ||
                                        type == VIR_PERF_EVENT_MBML)) {